Ahamadya is not Islam but it is against Islam simply because it was first appeared to empty Islam from the basics of Islam, it was also formulated to serve the imperialistic goals in India. Please not Prophet Muhammad hold any innovation in Islam abhorrent and even leaving the religion. Islam is a complete system that prophet finalized it shortly before his death.
In these slides you will read about some of the practices and beliefs that contradict holy verses from Quran and surely Islam denies.
Qadianism (Ahmadiyyat) is based on the belief that Mirza, the founder of the movement, was an improved second reincarnation of Muhammad(SAW